A hot melt adhesive is a non-solvent type adhesive which has a solid content of 100% at room temperature. It is heated and molten when used and adhered and applied on a matter to be adhered, followed by cooling to solidify it, whereby the adhesive strength is revealed. This hot melt adhesive has less risks of environmental pollution and a fire because of a non-solvent type and has such many advantages that setting time for adhesion can be shortened and handling is easy. Accordingly, it is used widely in various fields such as bookbinding, packaging and woodworking as applicators spread rapidly.
Hot melt adhesives are usually composed by adding various waxes as a tackifier and a viscosity control agent to thermoplastic resins such as, for example, olefin base copolymers which are base polymers. Generally used as the base polymers described above are ethylene-vinyl acetate copolymers and ethylene-acrylate copolymers out of the olefin base copolymers in terms of flexibility and costs. Further, widely used as the tackifier are natural resins and derivatives thereof such as rosin base resins and terpene base resins, and petroleum resins. Petroleum waxes, mineral waxes such as coal, and low molecular weight polyethylene waxes are used widely as the waxes.
Hot melt adhesives are widely used in the respective fields of automobiles, packing containers and structural materials, but the sufficiently high adhesive properties are not displayed to base materials comprising olefin base copolymers having less polarity as base polymers and base materials provided with surface coatings with silicon and wax. On the other hand, hot melt adhesives using thermoplastic rubber are excellent in a heat resistance and a cold resistance but have a high viscosity, so that specific applicators have to be used.
Usually, required to hot melt adhesives are many characteristics such as (1) low melt viscosity in order to improve workability and a wetting property to base materials, (2) high cold resistance and flexibility as well as a high adhesive strength, (3) high heat resistance in order to allow an adhesive strength to be revealed even at high temperatures, (4) short setting time and (5) high heat stability. However, if the melt viscosity is reduced, the heat resistance is lowered. On the other hand, if the heat resistance is elevated, not only the melt viscosity grows large but also the cold resistance and the flexibility are reduced. Thus, plural conflicting characteristics are required in hot melt adhesives. In order to allow these characteristics to be compatible with each other, the structural components and the composition proportions have been investigated, but it is difficult to allow the cold resistance to be compatible with the heat resistance. An object of the present invention is to provide a hot melt adhesive composition which is excellent in a heat resistance and a cold resistance.